The invention relates to a workpiece with a substrate of ceramic, metal or polymer, the substrate having a surface which is conditioned to form a stable connection with a polymer and which is provided with a silica layer and, on top of this, with a silane coupling agent. The invention also relates to methods for producing and for making use of the workpiece according to the invention.
For the production of dental crowns, it is known from U.S. Pat. No. 4,364,731 to sputter a silica layer onto a cleaned ceramic, metallic or polymeric substrate surface, to apply a silane coupling agent thereon, and finally to apply a polymeric end layer (xe2x80x9cplastic veneeringxe2x80x9d). Such a coating procedure has to be completed in one location and within a very short space of time since otherwise, if the silane coupling layer is left exposed, physical or chemical damage to the extremely sensitive surface can occur, for example as a result of abrasion, a macroscopic impurity, for example in the form of skin flakes, or chemical contamination of the reactive layer surface, for example by reaction with nitrogen, hydrocarbons or other air constituents. According to this method, it is not possible to store and transport the workpieces coated with silica alone and those provided with silane coupling agent, and with the polymeric end coating only taking place later and/or at another location.
In the case of a dental crown which has been veneered in this way, routine dental work does not demand any sterility in the sense of freedom from germs. It is usually simply cleaned with ethanol. The requirement for sterility of a dental crown would also not be consistent with practice, since a dental crown, after production by the dental technician, is packed without special requirements in respect of sterility. On arrival at the dentist""s surgery and before it is fitted, it is handled by the dentist and by his assistant, albeit with observation of certain hygiene procedures, but also be contact with the fingers. The nature of its handling thus rules out the continuation of any sterility that might be present. When fitted in the mouth, the crown is immediately colonized by numerous germs and other microorganisms typically found in the mouth, so that any previous sterility would be pointless. Even the healthy oral cavity is in fact xe2x80x9cphysiologicallyxe2x80x9d heavily colonized by germs at all times. This flora is important for digestion since the digestive process is initiated by the saliva in the mouth.
Also, in the case of such a dental crown, a polymeric coating does not in any way act as a protective layer against physical, mechanical or other stresses. It is instead an aesthetic veneer (typical thickness 1 mm) for in most cases a metal crown, and sometimes also for a ceramic or polymeric dental crown. Veneering with plastic is not intended to provide a protective function for the base of the crown. For example, there is no protection against:
a) physical abrasion: mastication function (the base of the crown itself is substantially more resistant to abrasion than the polymeric veneer) or
b) chemical influences: corrosive attack by saliva, food and medications, microbial effects, e.g. by acid excretion of bacteria (the base of the crown, in particular when noble metals or ceramics are used, is substantially more stable to corrosion than is the plastic coating), or
c) the influence of temperature changes when eating: these changes can, as a result of different coefficients of expansion, induce cyclically alternating stresses which impair the mechanical stability (the base of the crown is much more resistant to these cycles than is the veneer).
It should be noted that in general (in Germany) the base of the crown is made of an alloy with a high gold content. Ni and CoCr alloys are also commonly used (also known as superalloys from weapons technology), and also Pd-based alloys (only biocompatible to a limited extent; these economy alloys are controversial since they have a high allergy potential) or unalloyed titanium (except for the colour and the difficult processing, they are equal to gold alloys in terms of the above aspects). All the alloys are configured for oral stability and this is guaranteed in particular in the case of alloys having a high gold content. The plastic veneering as a measure for improving aesthetics does not provide additional oral stability; nor is this the intention. On account of their material properties, plastics are in fact less stable in principle in the mouth than are highly corrosion-resistant metals or ceramics.
The plastic veneers are primarily used for the temporary and removable tooth replacement (possibility of repair, given the high abrasion and cracking of the plastic).
The dental crown produced with the layer system of xe2x80x9csubstrate, silica layer, silane layer, plastic veneerxe2x80x9d thus represents a finished product which can be used in this form in the patient.